Research proposed here involves two interrelated lines of research on speech perception: the specification of the acoustic information that supports the perception of phonetic units and investigations of the characteristics of phonetic processing. The first line is concerned with a search for the dynamic invariants which carry information for vowels and those which carry the information for liquids, /r/ and /l/. Studies of the perception of both natural and synthetic speech across speaker differences, consonantal context, different speaking rates, different types of synthesis and electronic modification of digitally stored natural-voice syllables are proposed. Additional studies are concerned with sophisticated analyses of the acoustic signal, relating that analysis to perceptual data, and verifying findings through new synthesis. The aim is to find the abstract relations in syllables that explain the perceptual equivalence of acoustically diverse patterns. The second line of research examines the nature of the perception of speech syllables. The modifiability of phonetic perception under the impact of training procedures (short-term experience) and as a function of first-language acquisition (long-term experience) will be studied. Laboratory studies of the perceptual separability or integrality of distinctive features in natural syllables are also proposed. The emphasis throughout all these studies is on describing in detail how listeners can and do perceive speech. This research program will increase our basic understanding of the nature of speech perception by describing what acoustic information is perceived, how that information is processed, and how experience modifies those processes.